Our investigation into the morphogenesis of bacteriophage will be continued, with emphasis on three aspects: (1) the detailed mechanisms of T4 phage tail fiber assembly and attachment, (2) the role of the host cell in viral assembly, and (3) the mechanism of the DNA packaging reaction of T7 phage. (1) The assembly and attachment of T4 tail fibers requires 4 proteins under phage genetic control, which are not found in the completed structure. Biochemical and genetic approaches will be followed in attempts to understand the mechanisms by which these proteins promote or direct the assembly of the structural protein components of the tail fibers. (2) Bacterial mutants have been isolated that block the normal process of T4 assembly, or that allow the assembly of mutant phage that are defective in a normally essential gene. The existence of these mutants demonstrates the involvement of host cell factors in viral morphogenesis. We shall continue our genetic, physiological, and ultrastructural analysis of these mutants with the goal of understanding the roles of these factors in normal cell function and in phage assembly. (3) Using an in vitro system for T7 DNA packaging we plan to investigate the effects of varying reaction conditions and to characterize packaging intermediates in attempts to understand the mechanism of this reaction.